1. Field of the Invention
The present invention relates to an optical disc recorded-information reproducing apparatus and, more particularly, to an information detection light intensity control apparatus to control an intensity of an information detection light (rays of light incident upon an optical disc) which should enter an optical disc.
2. Description of the Related Arts
In an optical disc recorded-information reproducing apparatus to reproduce a signal recorded in a compact disc or a video disc in which pits corresponding to information signals to be recorded were recorded onto the surface of a disk-shaped recording medium, a laser beam is converged onto the pit or non-pit and an increase or decrease of a reflection light intensity is detected, and the signal recorded therein is read. In an information detection optical system of the optical disc recorded-information reproducing apparatus, there is a limit of a cut-off spatial frequency 2NA/.lambda. (NA denotes a numerical aperture of an objective lens and .lambda. indicates a wavelength of the laser beam). In order to raise dencity of a recording film surface by increasing a spatial resolution, it is necessary to increase the numerical aperture of the objective lens and to shorten the wavelength of the laser beam.
As an example of an optical disc recorded-information reproducing apparatus for raising the spatial resolution, an apparatus which uses an apodization or a super resolution has been proposed in Japanese Patent Kokai No.2-12625. In an irradiation optical system of such a disc recorded-information reproducing apparatus, a light intensity of circumference of the center of an incident pupil of a laser beam for reading is reduced by a shielding plate, a pattern spot is formed on the optical disc surface, and a main lobe in which a diameter of the spot is small is used.
In consideration of an intensity distribution of the laser beam which enters such an optical disc, in order to obtain a smaller and more effective diffraction image spot, an optical disc having a reflection film using a phase change material such as SbSe or the like in which a reflectance rises in association with an increase in temperature, that is, which indicates a reflectance temperature dependency, has been proposed in Japanese Patent Kokai No.3-292632. According to such an optical disc, since the reflectance in a beam spot for detecting information of the optical disc can be changed partially, an effective spot diameter of the beam spot for detection of information on the optical disc can be reduced. A spatial frequency which is equal to or higher than the cut-off spatial frequency of an information detection optical system can be therefore reproduced.
In what are called super resolution reproducing systems as mentioned above, disc recorded-information is read out by an information detecting system as shown in FIG. 1.
In FIG. 1, an emission light generated from a semiconductor laser 11 enters an optical disc 14 as an information detection light through a beam splitter 12 and an objective lens 13. The information detection light which entered the optical disc 14 is reflected by an information recording surface of the optical disc 14 and reaches the beam splitter 12 again via the objective lens 13. Such a reflection light from the optical disc 14 passes in the beam splitter 12 and is led to a photodetector 15. The photodetector 15 generates an electric signal in accordance with a light reception amount. The electric signal is supplied to a signal reproducing system (not shown).
A power monitor output terminal for outputting a signal indicative of an intensity of the emission light is extended from the semiconductor laser 11. A signal which is output from the output terminal, that is, an emission light intensity signal is supplied to one of input terminals of a comparing circuit 16 comprising, for example, an operational amplifier. A reference voltage V.sub.ref from a reference voltage generating circuit 17 is supplied to the other input terminal of the comparing circuit 16. The comparing circuit 16 compares the level of the emission light intensity signal with the level of the reference voltage V.sub.ref and supplies a signal according to the level difference between them to a driving circuit 18. The driving circuit 18 supplies a laser driving signal having a format to be supplied to the semiconductor laser 11 in accordance with the supplied signal thereto.
In the information detecting system, the semiconductor laser 11 is driven and controlled so that its emission light intensity is constantly set to a predetermined emission light intensity indicated by a preset reference voltage V.sub.ref.
In a system for reading information recorded on a disc in which an information recording surface is formed by a super resolution film having characteristics such that a reflectance changes according to a light irradiation as in foregoing Japanese Patent Kokai No.3-292632, a threshold value of the super resolution film is set to a predetermined light intensity of an information detection light which is generated by the semiconductor laser 11. The above point will now be explained on the basis of FIG. 2. FIG. 2 is a diagram showing an intensity distribution on the disc of an information detection light. An axis of abscissa (r) indicates a distance from the center of the information detection light. An axis of ordinate (I) indicates a light intensity level. A characteristics curve i.sub.0 drawn by a solid line indicates an intensity distribution of the information detection light in case of no aberration in the disc. From the diagram, it will be understood that the center of the information detection light indicates the maximum intensity. The value (I.sub.th) which is about 80% of such a maximum intensity value of the spot center of the information detection light is set to a threshold value of characteristics (irradiation light intensity I-reflectance R characteristics) of a reflection film of the super resolution optical disc as shown in FIG. 3. As will be understood from FIG. 3, the reflectance of the super resolution film has characteristics such that the reflectance rises by setting the threshold value I.sub.th as a border.
Therefore, when the information recorded in the super resolution optical disc in which the threshold value has been set as mentioned above is detected, an intensity portion which is larger than the threshold value I.sub.th is an effective spot for an information detection in the information detection light generated by the semiconductor laser 11.
In case of the information detection of the super resolution optical disc is executed in such an information detection system as shown in FIG. 1, however, when there is an aberration of a fluctuation such as disc tilt, disc thickness variation, or the like or a sensitivity fluctuation of the super resolution film, a spot head value of the information detection light is deviated from the optimum value, so that there is an occurrence of problem such that the information reproduction of the super resolution optical disc is unstable.
For example, in FIG. 2, a characteristics curve i.sub.1 drawn by a coarse broken line indicates an intensity distribution of the information detection light on the disc when a coma aberration in the track direction (tangential direction) on the disc is equal to 0.035 .lambda.RMS (.lambda. denotes a wavelength of the information detection light and RMS indicates Root Mean Square). A characteristics curve i.sub.2 drawn by a fine broken line indicates an intensity distribution of the information detection light on the disc when the coma aberration in the track direction (tangential direction) is equal to 0.07 .lambda.RMS. As the aberration increases, a peak power of the information detection light on the disc decreases by about 20% and there is a possibility such that the peak power is smaller than the threshold value I.sub.th in the worst case. In such an information reproduction of the super resolution optical disc, when the peak power level and the threshold value are close, a very unstable state such that the recorded-information light which should be reflected from the disc is not reflected is caused, so that it is a big problem.